The amount of goods transported by airfreight has increased considerably during the last years. In particular, transport of temperature sensitive goods, such as food, drugs, electronic equipment etc. has become more important. In order to be able to control the temperature even during the flight, insulated containers with temperature regulating equipment have been developed. Since security regulations concerning airfreight puts severe limitations on which type of equipment to be used during the flight. Carbon dioxide ice, placed in an icebox, is typically used as refrigerating medium. The icebox cools down the surrounding air and by means of small battery-driven fan systems, the cold air may be distributed within the container compartment.
In the design of ULD:s in general, such as air-cargo containers, weight is one of the most important parameters. Thus, the manufacturing of containers according to the prior art aims to reduce the amount of used material as far as possible. The icebox, fan equipment, control equipment and screens for defining airflow paths are thus attached directly into position in the container. The mounting takes place piece by piece, and makes use of the container walls and ceiling to reduce the total weight.
When the carbon dioxide ice evaporate, carbon dioxide gas is produced. This gas has to escape from the icebox. Some goods transported in cooled containers will be damaged by carbon dioxide exposure, why the carbon dioxide gas has to escape directly to the exterior of the container, and the icebox is therefore normally placed in the close vicinity of one of the container walls. The icebox also has to be sealed off from the interior of the container. Such sealing is performed directly against the container walls to ensure that no leaks will appear.
One example of an air-cargo container is disclosed in the international patent application WO97/27128 of Frigotainer AB. In order to increase the temperature interaction between the icebox and the surrounding air, an airflow path is typically arranged around the icebox, normally by providing flow paths in contact with the icebox. The typical configuration is to mount the icebox at a distance from the wall and ceiling of the container, using the container shell as the outer constriction of the flow path. This increases the sealing problems even further.
Containers according to the prior art have a number of disadvantages. First, the mounting times are long, since the difficult and detailed attachment of the numerous details of the cooling equipment takes place in a ready container shell. The limited space within the container makes the work difficult, and the total volume of the container requires large available mounting areas. The detailed mounting also requires skilled personnel and often also special tools, which makes end mounting and maintenance difficult at other places than at the production plants.
The distribution of containers from the manufacturing plant to the customers normally takes place by airfreight. Smaller numbers of empty containers may be fitted into free space in different transports, but there is no place available for larger amounts. Special freights then have to be arranged for the distribution of new empty containers, which increase the total cost.
There are also some minor technical problems with air-cargo containers according to prior art. The airflow path around the icebox is typically in direct contact with the container wall. The wall will thus be cooled at a section where the need of cooling is low, reducing the available cooling effect of the carbon dioxide ice. Furthermore, the cooling of the container wall and ceiling at this position may even cause condensation or ice build-up problems at the outside of the container.
Also, during periods, where no cooling of the container compartment is not necessary and any fans are shut off to stop the airflow around the icebox, there are problems with cold air flowing down from the icebox in the opposite direction than the intended one. In order to stop such back-streaming of cold air, the airflow path situated above the inlet and outlet openings of the airflow path may be thermally insulated in order not to produce any cold air. However, such insulation reduces the available heat exchange between the air and the icebox.